Revert "Revert "Revert "DebugInfo: Omit debug info for dynamic classes in TUs that do not have the vtable for that class"""

This reverts commit r188642.

This change is causing LTO builds to cause our 16 GB machines to swap and OOM
all weekend. I am going to work with Dave Blaikie to resolve the issue.

Sorry Dave =(.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188687 91177308-0d34-0410-b5e6-96231b3b80d8